Document transport systems are used in a wide range of document handling devices, including printers, copiers, scanners, fax machines and the like. In the design and implementation of document transport mechanisms, considerable attention is paid to handling documents to avoid damage, to prevent jamming or mis-positioning, to maximize throughput and efficiency, and to stack the output documents in a form that is convenient for operator handling.
The design of any particular document transport system is directly related to the larger purpose of the apparatus that processes the document. For printers and copiers, for example, the document transport system is designed for efficient handling of large volumes of paper documents, wherein the documents for any particular printer or copier job are most often of the same size and weight. Optimization of this paper path is generally directed to speed and efficiency of operation. Output feed mechanisms of such systems, for example, often propel the completed document outward so that it strikes a retaining wall or structure with some force, using this type of arrangement to provide a uniform stacking.
Facsimile machines and many types of low-cost office scanners are primarily designed for scanning images from standard letter-sized paper sheets (8.5×11 in. or metric A4). However, more capable scanners are expected to handle documents of various sizes, including stacks of documents in which successive documents can be of different weights and dimensions. For example, a governmental office may need to scan folders having mixed document content, such as licenses, title forms, canceled checks or credit card payment slips, signed forms, and miscellaneous communications, where documents in any folder can be of various sizes and weights. For handling mixed document stacks such as these, there are at least two particularly notable challenges for the document transport apparatus:                (i) Must handle documents of different size, such as length, and weight in succession as part of the same scanning job. With documents of inconsistent size and weight, the document transport apparatus must handle and provide these documents in an orderly, consistent manner.        (ii) Must preserve the document stacking order. This is increasingly important, particularly, for example, where a stack of documents relates to a property transaction, dispute or procedural process, or other legal matter. With many document stacks, the scanner must provide, in its output tray, the stacked documents in the same document stacking order as they were placed in the input tray. If this is not done properly, an operator may be required to re-sort documents back into order manually, which adds cost and the risk of operator error.        
Although the problems described in (i) and (ii) have been recognized by scanner designers, existing approaches for their solution have been disappointing or constraining due to mechanical complexity and expense. Some scanners, for example, employ the conventional leading-edge output stacking arrangement that is often used by high-speed copiers. Although such a system can be adapted so that it handles documents of different size and weights as just described in (i), it can perform poorly as related to requirement (ii), often failing to maintain the order of documents, particularly where smaller documents such as checks, index cards, or deposit slips are interleaved with larger documents.
One type of solution that has been used for maintaining document order is a stacker, such as the movable document stacker used in a copier and described, for example, in U.S. Pat. No. 6,378,864 entitled “Stacker” to Iesaka. However, stackers add considerable expense and mechanical complexity, and often require careful operator handling in order to put the document stack back into correct order. Another solution that has been used for maintaining stacking order with a stack of mixed-size documents is a stacking wheel, sometimes termed a “money wheel”, that receives a document, rotates at least partially to flip the document, then cooperates with a device that strips the document from the wheel and drops it onto an output tray or elevator. As the very description suggests, the stacking wheel approach can be mechanically complex and relatively costly, and adds to the size and overall mechanical footprint of the scanning system.
U.S. Pat. Nos. 4,959,685 and 6,626,428 describe sheet ejection mechanisms for image forming apparatuses wherein imaged sheet ejection speeds may be controlled to be different from sheet transport speeds in other sections of the apparatus. There is no teaching with respect to a scanner apparatus capable of handling a stack of multiple documents of different sizes and weights and of maintaining the order of such a stack of documents at the conclusion of a scanning process.
Thus, it is seen that there is a need for a document transport apparatus that is capable of handling multiple documents of different sizes and weights and of maintaining the document stacking order at the conclusion of the scanning process.